The present invention relates to a process for preparing a multicolored proof for the graphics industry and to a radiation-sensitive recording material which is suitable for use in this process and comprises, in general, a sheet-shaped temporary support, a thermally transferable layer containing a colorant and a radiation-sensitive layer disposed on top.
Color proofing is, in general, carried out in multicolor printing to permit corrections of the color separations used in exposing printing plates. The proof must be a consistent reproduction of the desired half-tone or line image and should neither increase nor reduce the tonal value of colors. Visual color proofing should reveal defects in the original, offer the best color rendition to be expected from press printing, produce the exact gradation of all color shades and indicate whether gray tones are neutral. It should furthermore indicate the need, if any, for subduing any of the colors and/or give directions for altering the film original before making the printing plate.
In multicolor printing, proofing has hitherto been made with the aid of colored press proofs. For this purpose, all the steps necessary in actual multicolor printing must be taken. A color proofing process of this kind is costly and time consuming. Other color proofing systems have therefore been developed, which are intended to have an equally good quality as press proofs.
To prepare color proofs which are consistent with the press prints to be made (print screen, color value similar to that of the printing ink) photochemically and electrophotographically working systems are, up to the present, the only systems which have found general acceptance.
Besides, there are physical methods of image generation, for example, ink jet, thermal transfer and thermal sublimation processes, which may give good colored images, but do not show a print screen and are therefore not comparable to color proofs which correspond exactly to the later press prints.
A great number of photochemically working color proofing systems are known in the art. The so-called "overlay systems", as described, for example, in GB-A 1,143,679 or U.S. Pat. No. 4,489,153, have the disadvantage that the proof is built up from four separate films by superposing partial images in the four primary colors and that, consequently, the impression gained from the image can be severely distorted by the inherent coloration of the films.
Further development in the field of color proofing led to composing an image on a support by way of several laminating steps. The final colored image is generated according to different methods. In the layer transfer processes, for example, according to U.S. Pat. No. 4,869,993 and EP-A 0 035 028 (=U.S. Pat. No. 4,260,673), the individual, unexposed layers are transferred to a support using an adhesive layer in each case and are there exposed and developed, one after the other.
In the image transfer systems as described, for example, in DE-A 19 12 864 (=U.S. Pat. No. 3,721,557) or DE-A 38 13 722 (=U.S. Pat. No. 4,980,260), exposure and development are carried out prior to the laminating step and the partial images in the primary colors are then composed on a support by means of an adhesive layer.
Processes are also known, in which uncolored photosensitive layers are laminated to a support, exposed under the individual color separations and optionally developed. In the process, differentiation between sticky and non-sticky layer areas occurs in the layer surface. A colorant powder is, in each case, adhered to the sticky layer areas to produce partial images in the corresponding primary colors. Processes of this kind are described, for example, in U.S. Pat. No. 4,356,253 or EP-A 0 320 900.
All the known laminating processes have the serious disadvantage that, irrespective of the fact that the proofs so produced come close to a printed image, the image reproduction is impaired by the presence of adhesive and/or polymer layers containing the image elements of the partial images in the primary colors embedded therein, as compared with a printed sheet, in which the individual ink dots are present directly on the printing substrate.
To overcome this disadvantage, EP-A 0 277 038, for example, describes a process, which comprises removing the polymer and/or adhesive layers present in the non-image areas in a developing step. In the image areas, however, the adhesive and/or polymer layers remain intact which, again, leads to an impaired image reproduction.
GB-A 2,213,950 describes a process which operates without adhesive layers. A photopolymerizable, dyed layer is exposed imagewise and then contacted with a printing substrate. Taking advantage of the difference in tackiness, the tacky dyed layer areas which have not been completely exposed are transferred to the printing substrate with the application of pressure and heat.
This process has the disadvantage that its application is limited to photopolymer systems and, what is more, photosensitive layer and dyed layer are combined. It is therefore impossible to use photoinitiator systems which have an adequate absorption in the visible wavelength region.
Electrophotographically working color proofing systems which have recently been propagated and are described, for example, in U.S. Pat. No. 4,510,223 and EP-A 0 236 783 eliminate this deficiency, but require considerable expenditure for equipment and process and are thus very costly.